The present invention relates to the production of expanded foam articles, such as buoys and other products where foamed core characteristics are considered desirable or necessary.
More specifically, the invention relates to an improved method and apparatus for forming products, and the products therefrom, including but not limited to buoys and other flotation devices having an expanded foam interior, and a "coating" or exterior shell comprised of a thermoplastic resin. In one exemplary embodiment, a rotomolded buoy is formed, having a polystyrene foam interior core and a cross-linked resin shell.
It is known in the prior art to produce expanded polystyrene foam products, including ring buoys, wherein the product is provided with an integral, protective coating simultaneously with the process of molding the expanded foam interior body portion of the product. For example, in U.S. Pat. No. 3,309,439, a rotatable casting mold is charged with a molding composition comprising expandable polystyrene beads, a hydrate solution, and a resin powder. During rotation casting, the denser resin gravitates outwardly to a position against the mold walls to form an external coating, while the less dense polystyrene beads expand in the interior portion of the mold to form the foamed interior of the product. This technique has not been completely satisfactory however, since good separation between the outer shell or skin and inner foam compositions is not always obtained. The result is an external skin with a splotchy, unattractive appearance caused by mixing, or incomplete separation, of the expandable polystyrene and the resin.
In addition, because the foam charge requires an elevated temperature to cause expansion of the polystyrene beads, outer shell deformation may occur, since most commercially acceptable resins are soft and somewhat pliable at such a temperature. Attempts to solve this additional problem by lowering the temperature have not been successful because there is then insufficient heat to insure optimum expansion of the polystyrene beads.
This invention relates to a method and apparatus which provides an effective solution to the above mentioned problems, and to products produced by the method.
In accordance with this invention, a thermoplastic resin is employed to form the outer shell or skin of a rotomolded product and an expandable resin is employed to form a foam interior. The manufacturing process is carried out in two separate steps wherein the outer skin or shell is rotation cast or molded in an initial step and, after curing of the shell but still at an elevated temperature, a foam charge is released within the mold in a second step, while the mold continues to be rotated. The finished product removed from the mold has been found to have a uniform appearance, with no significant mixing of the shell and foam compositions, and with no deformation or irregularities in the exterior surface of the product.
In carrying out the process of the invention, means are provided for holding and releasing the foam charge within the mold during the rotation casting, or rotomolding, process. The mold itself, which in one exemplary embodiment is spherical in shape, may be of conventional design, with a parting line about the center of the sphere. A conventional mold can be adapted quite easily in certain aspects to accommodate the foam charge holding and releasing means.
Further, the mold can be adapted to accommodate, for example, a cross bar which can be utilized in this invention as a carrier for the foam charge holding and releasing means. The holding and releasing means may include a container or containers, which may be cylindrical, rectangular, or some other suitable shape, and which can be attached to the the cross bar by any suitable, conventional attachment means, such as a bracket, strap, adhesive tape or the like. One or both ends of the container(s) may be provided with a removable lid, which can be secured to the container by a suitable means such as a heat resistant filament-reinforced tape. The tape may extend through a respective slot formed in one of the mold halves to enable the casting machine operator to separate the container(s) and lid(s) without having to open the mold itself. Under certain molding conditions, however, the expansion of the foam charge itself is sufficient to open the container(s).
Alternatively, the foam charge container(s) may comprise a plastic bag(s) which ruptures as the foam charge expands.
In its broader aspects, the invention also relates to a method of forming a rotomolded product comprising the following steps:
(a) charging a casting mold with a first charge containing a thermoplastic resin composition; PA1 (b) placing within the casting mold a second charge, isolated from the first charge, the second charge containing an expandable foam composition; PA1 (c) rotation casting within a heated chamber to cause the resin composition to flow about the mold walls and to fuse thereto as an outer shell; PA1 (d) after fusion of said resin composition, but while the shell is still heated, releasing the foam charge within the mold for expansion within the heated shell to form the foamed interior.
Utilization of the above described method and associated apparatus provides a high quality product having a smooth and uniform exterior surface, a true exterior shape, and excellent foam core characteristics.
In one specific exemplary embodiment, a flotation device, such as a buoy, may be constructed by the disclosed method and apparatus. The outer shell of the buoy comprises polyethylene containing a cross-linking agent, and the foam charge comprises polystyrene beads. The mold is provided with apertures for supporting a cross bar which protrudes from opposite sides of the mold. The cross bar functions not only to hold the foam charge within the mold, but also to provide ready attachment means for hardware such as mooring rings, on the finished buoy. In this regard, a mooring ring and associated washer may be attached to one end of the cross bar prior to commencing the molding process, and the other ring and associated washer may be attached after the buoy is removed from the mold.
The cross bar in this particular embodiment supports a pair of styrene containers or cups and associated separable lids. More specifically, the lids may be secured e.g., by spot welding, to the cross bar in back-to-back relationship, and the respective containers temporarily held in place with their associated lids by a plurality of strips of fiberglass reinforced adhesive tape.
Subsequent rotation casting within a heated chamber causes the polyethylene composition to flow about the mold walls and to fuse thereto, rendering the polyethylene cross-linked. After fusion of the polyethylene, but while the shell is still heated, the adhesive on the fiberglass tape softens permitting separation of the containers from their respective lids as the mold continues to rotate, thereby releasing the polystyrene beads for expansion within the shell. Upon completion of the forming process, the finished buoy is removed from the mold and mooring ring and washer are fastened to the other end of the cross bar.
It will, of course, be understood that this same basic process can be used to form a number of foamed core products utilizing a number of suitable resin shell and foam core materials. For example, resin shell materials may include polyethylene, polyvinylchloride, polypropylene, nylon, acrylonitrile-butadiene-styrene (ABS), and polycarbonate. In addition, thermoplastics such as polyethylene, polyvinylchloride and nylon which contain cross-linking agents may also be used.
Other objects and advantages of the invention will become apparent from the drawings and detailed description which follows.